Various positive electrode materials have been developed as positive electrode materials of lithium ion batteries as secondary batteries. Particularly, in recent years, instead of lithium cobaltate that has been used conventionally, a nickel-cobalt-manganese (NCM)-based positive electrode material that is called a ternary positive electrode material, a nickel-cobalt-aluminum (NCA) positive electrode material that is called a nickel-based positive electrode material, or the like is drawing attention.
The positive electrode material containing nickel as described above is produced, for example, by treating a solution containing a salt of metal such as nickel with alkali and subjecting the obtained metal hydroxide to a calcination treatment. Such a metal salt is produced, for example, in a nickel smelting step using a nickel oxide ore or the like as a raw material, and specific examples thereof include a chloride (nickel chloride) and a sulfuric acid salt (nickel sulfate). Incidentally, of them, in the case of using a chloride, when a hydroxide obtained by neutralizing a chloride is calcined, remaining chloride ions become chlorine gas and this chlorine gas may cause corrosion damage of a firing furnace. For this reason, in general, a sulfuric acid salt is used as a metal salt in many cases.
Herein, the nickel sulfate is obtained as a by-product of a step of smelting electrolytic nickel from a nickel oxide ore. However, since cobalt is also contained in the nickel oxide ore in many cases and cobalt is also co-precipitated in the electrolytic nickel, the quality of the electrolytic nickel is degraded; meanwhile, the recovery loss of cobalt as a valuable metal may occur.
For this reason, nickel and cobalt are separated using a wet treatment such as a solvent extraction method in the smelting process, but since those metals have similar chemical properties, it is not easy to separate each metal, and lots of cost is required.
Incidentally, a positive electrode material such as an NCM-based positive electrode material or an NCA-based positive electrode material is formed from a composite metal oxide containing nickel and cobalt. That is, in the aspect of nickel smelting, if nickel sulfate containing cobalt as impurities is used without change as a raw material for producing an NCM-based positive electrode material or an NCA-based positive electrode material, it is not necessary to separate nickel and cobalt, so that the nickel sulfate may be a material that is advantageous in terms of cost.
However, in the aforementioned nickel sulfate, calcium derived from a neutralizing agent to be added in the process of smelting electrolytic nickel from a nickel oxide ore or calcium existing in the nickel oxide ore itself as a raw material may be contained. Further, when a positive electrode material such as an NCM-based positive electrode material or an NCA-based positive electrode material is produced using such nickel sulfate containing calcium as a raw material, calcium is contained as impurities in the electrode, and according to this, battery characteristics such as charge and discharge capacity of a lithium ion battery may be largely degraded. Therefore, in order to use nickel sulfate containing cobalt as a raw material for producing an NCM-based positive electrode material or an NCA-based positive electrode material, it is important to efficiently and easily remove calcium as impurities.
As a known method for removing such an impurity metal, methods such as a precipitation method, a cementation method, a crystallization method, and a solvent extraction method are exemplified.
Of them, the precipitation method is to precipitate metal ions to be removed as a sulfuric acid salt or a hydroxide and then remove the metal ions. However, calcium ions cannot be precipitated as a sulfuric acid salt. Further, in the case of precipitating calcium ions as a hydroxide, it is necessary to prevent co-precipitation of components to be recovered such as nickel and cobalt; however, under the alkali condition of a high pH for precipitating calcium as a hydroxide, nickel and cobalt are also precipitated as hydroxides. Therefore, it is difficult to separate calcium from nickel and cobalt by such a precipitation method.
Further, the cementation method is a separation method using the phenomenon that, in a case where metal ions exist in an aqueous solution, when a metal having a lower oxidation-reduction potential than that of a metal existing as ions is added, exchange of electrons is performed between the metal ions and the added metal, the metal ions are reduced to metal and precipitated, and the added metal is oxidized and dissolved as ions. However, since a standard oxidation-reduction potential of calcium is lower than a standard oxidation-reduction potential of hydrogen, even in the case of adding a lower oxidation-reduction potential than calcium, protons are reduced so that calcium is not reduced. Therefore, calcium cannot be removed even by the cementation method.
The precipitation method and the cementation method as described above are methods for precipitating a metal to be removed in the aqueous solution and remove the metal; on the other hand, the crystallization method is a method for heating and condensing an aqueous solution to precipitate a salt of nickel sulfate or cobalt sulfate and remaining impurities in a mother liquid for crystallization to perform purification. However, in this method, since a solution containing sulfuric acid ions is used, calcium reacts with sulfuric acid ions, and thus hardly soluble gypsum (CaSO4.2H2O) may be formed. Therefore, when the level of concentration of a metal is increased in order to recover nickel and cobalt with a high recovery rate, the calcium concentration is also increased inevitably, so that a possibility that gypsum is formed increases. On the other hand, when the formation of gypsum is tried to be suppressed, the level of concentration of the metal cannot be increased, so that it is not possible to obtain nickel and cobalt with a high recovery rate. Further, in the method by the crystallization method, cost required for heating for condensation is also increased.
Meanwhile, the solvent extraction method is a method for extracting impurities in an organic solvent and removing the impurities, and by appropriately setting an extractant and an extraction condition, impurities can be selectively removed. As a method for removing calcium from a nickel sulfate aqueous solution using the solvent extraction method, for example, Patent Document 1 proposes a method for removing calcium to be dissolved in an electrolyte of nickel using alkylphosphate ester as an extractant. Specifically, by using alkylphosphate ester as an extractant and adjusting the pH of the nickel solution at the time of extraction to 1.5 or more and 5.0 or less, extraction and removal of calcium as impurities from the solution are performed. In particular, by adjusting the pH of the solution at the time of extraction to 4.0, the content of calcium in the nickel sulfate solution can be reduced to 50 mg/L or less.
However, in the method described in Patent Document 1, when the pH of the solution is adjusted to around 4.0 at which the content of calcium in the nickel sulfate solution is reduced, cobalt as a rare metal is also extracted and removed at the same time, so that cobalt cannot be effectively used. That is, for example, upon producing a positive electrode material such as an NCM-based positive electrode material or an NCA-based positive electrode material, it is necessary to separately prepare a supply raw material of cobalt, so that production cost increases.
Further, as described above, in production of a positive electrode material of a battery, a mixed solution of nickel sulfate and cobalt sulfate is demanded, but in production of a plating material or a catalyst, a demand for a high-purity nickel sulfate solution is large. By producing only a mixed solution of nickel sulfate and cobalt sulfate of which use application is limited almost to production of a positive electrode material of a battery, a demand with respect to production of a plating material or a catalyst cannot be satisfied.
From such a point, a process by which both a mixed solution of nickel sulfate and cobalt sulfate and a high-purity nickel sulfate solution can be efficiently produced is demanded.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2012-072482